The soil of the Sonian Forest

The soil of the Sonian Forest has gone untouched for over ten thousand years. That is unique in North and Middle Belgium. Nowhere else will you find such a vast forest that saw no agriculture and only minimal grazing. “The Sonian soil is a particularly valuable historic document and a natural heritage that we have to treasure,” stresses soil expert Roger Langohr.

The Sonian Forest has some of the fastest-growing beech trees in the world. The growth rate in the forest is exceptionally high. Why is that? The explanation can be found underneath the ground of the forest.

“To understand nature, you mainly need to account for three elements,” explains soil expert Roger Langohr. “Climate, soil and history. The forest lies in the middle of the Belgian loam area, which stretches out over Middle Belgium, from Mons to Liège and from Mechelen to Charleroi. Ever since Roman times, it has been a very popular area for agriculture. Only the Sonian Forest remained untouched for all those years. It makes the forest one of the very few places in North and Middle Belgium where the soil has never been ploughed.”

Digging

We have the counts of Leuven and later on, the counts of Brabant to thank for the untouched Sonian soil. Up until the 18th century, they used the forest as hunting grounds and for wood and coal production. If you check the forest regulations of that period, you will learn that grazing was allowed, but limited. Roger Langohr: “From the Middle Ages until 1830, many forests served as pasture for sheep, pigs, horses and cows. But livestock affects the soil. Their dung brings more nitrogen into the soil, which attracts earthworms and moles. Those dig through the upper layer. That scarcely happened in the Sonian Forest, so the original soil profile remained intact.”

Fertile

If a place has ever been used for agriculture, the effects are still visible centuries later. Roger Langohr: “For example, the Romans used the Meerdaal forest near Leuven for farming. You can still see as much today in the soil profile. The upper humus layer is thicker and more fertile than the humus layer in the Sonian forest. There, the upper layers are very poor in nutrients and therefore very acidic. The acidity is among the highest in Belgium, even higher than on the Ardennes plateaus. The trees in the Sonian Forest grow as well as they do because of the meters-wide layer of lime-rich loam that was deposited here during the ice age. This layer is very fertile and criss-crossed by tree roots.”

Also unique: the Sonian Forest saw almost no erosion throughout the centuries. This means that the landscape has remained virtually intact. You can still see the meanders created after thaw set in during the ice age. Roger Langohr: “Nowhere else in the loam area of Western Europe will you find such an extensive variety of landscapes in one place: valleys, hills, plateaus…”

Threat

The soil of the Sonian Forest is unique, but that also makes it vulnerable. “The largest threat isn’t climate change. Instead, it’s people setting foot upon the soil or forest exploitation with large machines,” Roger Langohr says. “For example, in the seventies horse riders were allowed to ride between the trees in specific parts of the forest. During the dry summer of 1976 foresters noticed that several trees were dying, particularly in those parts. Turns out, due to the trampling of the horses, the soil had been compacted. Water and air couldn’t get to the roots anymore, and the trees died off. When they discovered this, the foresters immediately prohibited leaving the paths. But once the soil has been compacted, it needs decades to recover. That is why it is important that visitors to this magnificent forest don’t stray from the paths. It is a good thing that the foresters are taking measures right now to prevent the harmful consequences of recreation and forest exploitation.”

Vertical cross-section of a loam soil in the Sonian Forest

1) Layer of dead leaves
2) Thin layer of humus
3) Layer with several roots between 30 and 40 centimetres deep.
4) ‘Cracks system’ with vertical veins between 40 and 120 centimetres deep. The tree roots have to get through these small veins to get to the deeper-lying loam layer.
5) Bottom layer from 120 centimetres to 4 metres deep. Here, the roots reach the lime-rich, fertile loam that was deposited during the ice age. This layer is the reason why the Sonian trees grow so well.